214 HUMAN EMBRYOLOGY AND MORPHOLOGY. 



the Ehinencephalon, separating the uncinate gyrus from the parts 

 derived from the olfactory lobe. 



The student is already familiar with the fact that the Island 

 of Eeil forms a cortical cap to the corpus striatum. The struc- 

 tures between the islandic cortex and the foramen of Monro 

 represent a section of the thickened wall of the cerebral vesicle 

 (Fig. 169). Convolutions appear on it at the 7th month, when 

 the rest of the cortex also becomes convoluted. Further, the 

 larger the area of cerebral cortex in any primate, the larger is 

 the Island of Eeil; the more convoluted the cortex, the more 

 convoluted the Island. Flechsig has shown that the cortex of 

 the Island is joined to all the cortical areas of the mantle by 

 bands of association fibres. Hence the Island must be regarded 

 as playing a highly important part in co-ordinating the functions 

 of the brain. 



sup. preeentral "PP er R olandio 



post limb. fis. of. Sylv. 



1st temp. fis. 



temp. op. 

 orbit, op. islandic area 



Fig. 176.— The same Aspect during the 7th month. 



The Opercula. — Three opercula grow up and cover the Island 

 of Eeil (see Figs. 175 and 176): (1) the temporal, (2) the 

 fronto-parietal, (3) the orbital. Cunningham, whose researches 

 into this region of the brain have become classical, found that 

 during the later months (7-9) of foetal life, the orbital operculum 

 in quite 50°/ o of brains shows a subdivision into two, an upper, 

 the pars triangularis, and a lower, the pars orbitalis (Figs. 176 

 and 177). The subdivision occurs more frequently on the left 



